1. Field of the Invention
This invention relates to a method for starting up a reactor for the catalytic gas phase oxidation to be induced by the supply of a raw material to be oxidized in combination with a molecular oxygen-containing gas, and more particularly to a method for starting up a reactor for the catalytic gas phase oxidation, characterized by causing the raw material and the molecular oxygen-containing gas supplied to the reactor for the purpose of oxidation therein to pass a range in which the concentration of the raw material falls short of the concentration of the lower explosion limit of the raw material and the concentration of the oxygen is not less than the limiting oxygen concentration, excluding the concentration of the raw material of 0 vol. %.
2. Description of Related Art
The (meth)acrylic acid which is a general-purpose monomer is produced by the reaction of catalytic gas phase oxidation of propylene, isobutylene, t-butanol, methyl-t-butyl ether, acrolein, or methacrolein. Since this production consists in the oxidation reaction, it necessitates supply of a molecular oxygen-containing gas in combination with the raw material gas. And the oxidation reaction is generally an exothermic reaction by nature, the reactivity of the raw material gas is largely varied by the property of the catalyst to be used, the concentration of the raw material gas and the molecular oxygen-containing gas, and the conditions for the removal of the heat of reaction. When a general-purpose monomer is to be mass-produced on account of a large demand, therefore, it is important for the purpose of securing the largest possible yield of production to attain the efficient production by setting the optimum reaction conditions in the initial stage of the production of the target compound by the reaction of catalytic gas phase oxidation.
The interior of the reactor for the catalytic gas phase oxidation, however, is a multicomponent system comprising the reaction product in addition to the raw material gas and the molecular oxygen-containing gas. The composition in the reactor changes every moment from the time the reaction is started till the time the steady state is reached. It is generally held that the optimum combination of the three elements, i.e. the property of the catalyst, the property of the explosion range of the reaction gas, and the removal of the heat of reaction, is important for the reaction of catalytic gas phase oxidation. While these three elements are easily maintained after the reaction has reached a steady operation, it is extreme difficult for the elements to control between the time the reaction of catalytic gas phase oxidation starts and the time the reaction reaches the steady state. The reason for this difficulty is that the concentration of the raw material to the interior of the reactor is varied from a low level to a high level, that the amount of the heat generated in consequence of the oxidation reaction is changed in accordance with the fluctuation of the concentration of the raw material, and that the other elements which have direct influences on the reaction of catalytic gas phase oxidation are varied in consequence of variations of the elements mentioned above. From the time of starting the reaction till the time of reaching the steady state, therefore, it is necessary to select within the shortest possible duration such setting conditions as excel in the economy inclusive of the energy of consumption rather by taking account of the time and safety required for enabling the reactor to acquire the steady state, the amounts of the raw material gas and other gas to be used therefor, and the amount of the energy spent for heating the raw material than by paying full attention to the efficiency of the reaction. Moreover, in the reactor for the catalytic gas phase oxidation of a raw material compound for oxidation liable to induce an exothermic reaction prone to explosion, it is extremely important to carry out the reaction under the conditions which warrant safety of a high degree enough to avoid inciting combustion or explosion due to the relation between the raw material and the concentration of oxygen.
Since the explosion range inherent in the reactor for the catalytic gas phase oxidation varies with the temperature, the pressure, and the kind of the inert gas to be used, it is necessary to find the explosion range by a preliminary actual measurement and then proceed to control the reaction so as to avoid the explosion range consequently found. The explosion range of the ordinary reactor for the catalytic gas phase oxidation will be described below with the aid of FIG. 4. In FIG. 4, the horizontal axis is the scale for the concentration of oxygen and the vertical axis the scale for the concentration of a raw material to be oxidized and the hatched part is the explosion range which is formed by mixing oxygen and the raw material. The lowermost concentration of the raw material in the gas composition forming the explosion range is called “the concentration of the lower explosion limit of the raw material to be oxidized” and the lowermost concentration of oxygen in the gas composition forming the explosion range is called “the limiting oxygen concentration”, in the specification. With reference to FIG. 4, the point of intersection of these two concentrations in the reactor for the catalytic gas phase oxidation is indicated by the mark {circle around (3)}. When the composition of the feed gas in the steady state is indicated by the mark {circle around (1)} indicating the concentration of the raw material gas to be 4.5 vol. % and the concentration of oxygen to be 10 vol. %, for example, it is safe for the purpose of using the reactor while avoiding the explosion range and easy for the adjustment of the composition of the raw material as well to supply to the reactor a feed gas of the composition through the mark {circle around (2)} in which the concentration of the raw material gas is 0 vol. % and the concentration of oxygen is less than the limiting oxygen concentration. It has been heretofore customary, therefore, to adopt the route in which the concentration of oxygen passes the point {circle around (2)} which falls short of the limiting oxygen concentration regarding oxygen concentration and alter the composition on one straight line of the target composition marked as {circle around (1)}.
When the reactor is started up by the conventional method, however, since the reactor prior to starting the use thereof is filled with air, it is necessary for the purpose of lowering the concentration of the oxygen contained in the gas supplied to the reactor in the range of “less than the limiting oxygen concentration” to supply such so-called diluting gases as nitrogen gas and carbon dioxide gas in a large amount to the reactor and control the concentration of oxygen. Further, the operation of starting up the reactor does not merely reside in one course leading up to the steady state but even embraces in its object the evaluation of the fact that the operations of the reactor and the attached devices thereof are carried out safely and stably. It is, therefore, favorable to supply the same amount of the gas to the reactor during the course of starting up the reactor as when the reactor is in the steady state, with the result that the amount of the diluting gas to be used for controlling the concentration of oxygen will be increased. The use of the diluting gas in a large amount, however, is not economical because this gas is expensive.
Incidentally, the reaction of catalytic gas phase oxidation generally entails a step of absorption which absorbs the target compound included in a reaction gas into an absorbent and, at the same time, separates an discharged gas. The discharged gas which emanates from the absorption column contains nearly no object compound and, after the reaction is started in the reactor, the oxygen concentration in the gas changes lower in consequence of the consumption of oxygen by the reaction. Thus, after starting the reaction, this discharged gas can be recycled to the reactor in the place of the diluting gas with a view to saving the diluting gas to be introduced into the reactor. On the other hand, the control of the concentration of oxygen cannot be attained with the recycled gas prior to the introduction of the raw material to be oxidized. It still requires supply of the diluting gas in a large amount. Even after the introduction of the raw material to be oxidized, the replacement of the diluting gas in a large amount with the recycled gas necessitates changes and adjustments in the relevant flow volumes of such gases and, as an inevitable consequence, elongates the duration of the relevant step and enlarges the amount of the diluting gas to be supplied. In this connection, the method which evades the wide changes in the flow rate by having the reactor displaced in advance with a diluting gas is conceivable. Since this displacement consumes a long time and necessitates supply of the diluting gas in a large amount, this method does not deserve to be rated as an excellent measure of improvement.
All these methods are invariably uneconomical because they require supply of the diluting gas in a large amount and because the diluting gas they use is expensive. There are methods which use steam as a diluting gas. They are likewise unfavorable because they require supply of thermal energy in a large amount for the generation of the steam. An effort to save the diluting gas is likewise at a disadvantage in inevitably entailing an elongation of time.